As illustrated in FIG. 9, a conventional printing unit of a digital printing press includes plate cylinder 2, which carries printing film 1 therearound and rotates, ink feeding part 3 for feeding ink on the printing film 1 on the plate cylinder 2, rubber cylinder (not shown) onto which ink is transferred from the printing film 1, and an impression cylinder (not shown) for pressing and holding a printing sheet with the rubber cylinder so as to enable the ink to be transferred from the rubber cylinder to the printing sheet.
The printing press of FIG. 9 is so structured that the printing film 1 having such a length as to enable several images to be formed thereon is rolled up and set to the plate cylinder 2 for easy replacement of the printing film 1. Specifically, the plate cylinder 2 forms on its circumference recess 4, in which a roll of the printing film 1 is mounted, and includes feed reel 5 for feeding the printing film 1 onto the circumference of the plate cylinder 2, and rewind reel 6 for rewinding the printing film 1 by holding the leading edge 1a thereof so as to draw the film from the circumference of the plate cylinder 2.
Once the printing film 1 is set on the plate cylinder 2, image forming device 7, which is secured in place with a predetermined clearance to the circumference of the plate cylinder 2, forms an image on the printing film 1 placed on the circumference of the plate cylinder 2 so as to use the printing film 1 as a printing plate. The ink feeding part 3 feeds ink on the printing plate (the printing film 1) or performs any other operations so that the image is printed on the printing sheet via the rubber cylinder.
Subsequent to completing series of printing operations, the rewind reel 6 rewinds the printing film 1, which was used as the printing plate, so as to draw the same from the circumference of the plate cylinder 2, while the feed reel 5 successively feeds a new printing film 1 onto the circumference of the plate cylinder 2. That is, in the printing press having the printing unit as illustrated in FIG. 9, an image can newly be formed on the printing film 1 fed from the feed reel 5 by the image forming device 7, thereby enabling repeated printing operations without the necessity to replace the plate every time the printing operation is performed.
In a case where color printing (multi-color printing) is performed by using the aforesaid digital printing press, plural printing units as illustrated in FIG. 9 are provided. The printing unit of FIG. 9 is designed for a two-color printing, and therefore two printing units are provided to make up a digital printing press, in a case where the printing operation is performed by having four colors, namely C (cyan), M (magenta), Y (yellow) and K (black) overlapped to each other.
In the thus arranged digital printing press, image data (color separated, four image data) for the respective printing plates used in the printing units are created, thereby forming a printing plate in each printing unit based on each image data.
Image data for the respective printing plates must be prepared so as to achieve correct alignment of colors during printing. For the correct alignment of colors in printing, marks called “register marks” for the alignment are generally printed in a space near the edge of a substrate, and image data for each printing plate contains data (register mark data) for laying out register marks. This register mark data is also utilized to provide marks in sheet cutting operation in addition to an operation to adjust the alignment of colors in printing.
FIG. 10 respectively illustrate image data for the respective printing plates used in a digital printing press according to a conventional technique. As illustrated in FIG. 10, in the conventional technique, all the image data for the respective printing plates contain register mark data T1-T8. Herein, FIGS. 10(a), 10(b), 10(c) and 10(d) illustrate image data created respectively for a C plate, a M plate, a Y plate and a K plate.
The respective printing plates are formed based on the image data created respectively for them in FIG. 10. FIG. 11 illustrates a print formed by using these printing plates. As illustrated in FIG. 11, print 100 containing an illustration (a logo), a color picture, text and register mark data T1-T8 is made (Japanese Patent Application Laid-open No. 2002-166523).
The digital printing press made up of plural printing units as described above provides alignment with high accuracy and therefore there is a case where register mark data is not needed when it is presented for the purpose of adjustment of alignment of colors in printing. In such a case, it is possible to create each image data that contains no register mark data. Accordingly, in this case, the printing plates are formed based on image data without register mark data, overprinting is made by these printing plates, thereby making the print 100 as illustrated in FIG. 11.
However, according to the above conventional technique, whether register mark data are contained or not, image data created for forming the printing plates are allowed to have only a uniform size. That is, the printing film 1 for use in forming printing plates provides a uniform size for all the printing plates, regardless of presence or absence of an image to be printed, the size or the like of an image for each plate. There is a case where the size of an image to be printed is very small depending on the plate, which results in inefficient consumption of the printing film 1.
For example, in each of the C plate (see FIG. 10(a)), the M plate (see FIG. 10(b)) and the Y plate (see FIG. 10(c)), a portion S with no data except for register marks written therein is caused. That is, if this portion S in each of the C, M and Y plates is not served to provide a register mark, which means that this portion is not used in printing by all means, the digital printing press according to the conventional technique inefficiently consumed the printing film 1.
The present invention has been conceived to address a problem associated with the above conventional technique. It is an object of the present invention to provide a data creating device for printing plates that optimizes image data for printing process by using a minimum amount of a printing film.